Oxygen is a commodity chemical in the industrial gas industry. It has numerous applications including waste water treatment, pulp bleaching, glass manufacturing, and steel manufacturing. One of the most common methods of oxygen production is by adsorptive gas separation. However, this technology is not competitive for large size oxygen plants (&gt;90 TPD O.sub.2). The technology of choice for this size range is cryogenic distillation of air. There is a need in the marketplace to produce oxygen in quantities&gt;90 TPD at low capital and energy costs by adsorptive gas separation.
There are two major categories of adsorptive oxygen production processes--pressure swing adsorption processes (PSA) and vacuum swing adsorption processes (VSA). The pressure swing adsorption processes carry out the adsorption (feed) step at pressures much higher than ambient and adsorbent regeneration at pressures close to ambient. The adsorbent beds go through secondary process steps, such as pressure equalizations, depressurizations, blowdowns, and purge or various combinations of these during the cycle. Typical of the O.sub.2 PSA processes are U.S. Pat. Nos. 3,430,418; 4,589,888; 4,650,501; and 4,981,499.
These processes tend to be energy intensive and more suitable for smaller oxygen plants producing less than 20 tons of oxygen per day and preferably less than 5 tons of oxygen per day. A subset of O.sub.2 PSA processes is a rapid pressure swing adsorption (RPSA) process. As the name implies, this process involves similar steps as a PSA process, but carries out these steps very quickly. Again, this process tends to be energy intensive and suitable for oxygen plants even smaller than O.sub.2 PSA's.
Primary reasons for high energy consumption in PSA processes are: (1) O.sub.2 recovery from these processes is low, and (2) the entire feed stream has to be compressed up to the adsorption pressure. These inefficiencies are somewhat circumvented in vacuum swing adsorption (VSA) processes. In these processes, adsorption is carried out at pressure slightly above ambient and adsorbent regeneration is carried out at sub-atmospheric levels. The adsorbent beds go through several secondary steps with the primary aim of increasing oxygen recovery and reducing adsorbent inventory per unit of product gas. Most commercial O.sub.2 VSA processes employ two or three adsorbers, a feed blower, vacuum blower, and possibly a product surge tank.
U.S. Pat. No. 4,917,710 describes a two bed O.sub.2 VSA process with a product storage vessel. Process cycle steps are: adsorption, cocurrent depressurization, simultaneous cocurrent depressurization and evacuation, evacuation, vacuum purge by product, vacuum purge by gas obtained in a cocurrent depressurization step, simultaneous pressure equalization and product repressurization, and simultaneous feed and product repressurization. Gas for product repressurization and product purge is obtained from the product storage vessel. Gas for pressure equalization is obtained from the bed on simultaneous cocurrent depressurization and evacuation step.
U.S. Pat. No. 4,781,735 describes a three bed O.sub.2 VSA process with steps: adsorption, feed to feed or dual end pressure equalization, cocurrent depressurization, evacuation, vacuum purge by gas obtained in cocurrent depressurization step, product repressurization from bed on feed step, simultaneous feed repressurization and feed to feed or dual end pressure equalization.
European patent application 0 354 259 outlines various options for a two bed O.sub.2 VSA process: adsorption, cocurrent depressurization, evacuation, pressure equalization with gas obtained in cocurrent depressurization step and feed repressurization. An option includes vacuum purge by product gas from the bed on adsorption step.
U.S. Pat. No. 5,015,271 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation or feed, countercurrent evacuation, simultaneous product to product pressure equalization and feed repressurization, or vacuum purge, simultaneous feed and product repressurization and feed repressurization.
U.S. Pat. No. 5,122,164 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation, countercurrent evacuation, vacuum purge, pressure equalization with gas from a bed undergoing cocurrent depressurization and product repressurization.
U.S. Pat. No. 5,223,004 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation, countercurrent evacuation, purge, repressurization with product and cocurrent depressurization gas from another bed and repressurization with product and feed.
U.S. Pat. No. 5,429,666 describes a 2 bed O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation, countercurrent evacuation, purge, simultaneous repressurization with feed gas mixture and cocurrent depressurization gas, and repressurization with several combinations of feed gas, product gas, and ambient air.
The above described processes are ideal for producing up to 90 TPD O.sub.2 from a single plant. There is an incentive to search for alternative processes for plant sizes greater than 90 TPD due to the following two factors:
1. Maximum vacuum pump size. The positive displacement Roots type blowers, which are typically employed in this application, have a maximum size of 30,000 ACFM. This is sufficient to produce up to 90 TPD O.sub.2. Beyond that, two vacuum pumps are required. PA1 2. Adsorber size. Typical O.sub.2 VSA processes achieve an adsorbent productivity of 1.0 to 1.5 TPD of O.sub.2 production per ton of adsorbent. At this productivity level, the maximum production which can be achieved from two or three 15 foot diameter adsorbers is 60 to 90 TPD. Adsorbent productivity is a function of the quantity of O.sub.2 produced per cycle and the length of time (T) required to complete each cycle. For a plant with a single vacuum pump, the minimum value of T is n times E, where n is the number of adsorbers and E is minimum practical evacuation time. E is typically 40 to 45 seconds and T is typically 80 to 90 seconds for a 2 bed process, 120 to 135 seconds for a 3 bed process. PA1 1. Beyond 90 TPD, two parallel VSA plants of the type described above are required. The cost is essentially double that of a single plant. By contrast, the cost multiplier for a double size cryogenic plant is 1.5 times. PA1 2. Unit power consumption of the cryogenic plant drops as the plant size increases, as more efficient compressors are employed, and opportunities for heat integration and power recovery are exploited. Unit power of the parallel VSAs typically does not drop, despite significant opportunities in the process for energy optimization. Table 1 summarizes the results of an exergy analysis on the two-bed process described in U.S. Pat. No. 5,429,666. PA1 The theoretical work of separation represents only 15% of the total power consumption. Vacuum and feed blower inefficiencies contribute half the lost work in the system. Lost expansion energy contributes another 33%.
The previously described O.sub.2 VSA processes are more economic than cryogenic technology, up to 90 TPD. Beyond 90 TPD, the advantage of O.sub.2 VSA declines. The reasons for this are two-fold:
TABLE 1 ______________________________________ Lost Work Distribution ______________________________________ Cycle/Sieve/Beds 34% Vacuum Blower 32% Feed Blower 18% Waste Stream 7% Vacuum Line, DP, DT 6% Feed Line, DP, DT 3% ______________________________________
Several processes employing two integrated vacuum pumps and four or more adsorbers have been proposed to overcome the 90 TPD single train limitation. U.S. Pat. No. 5,393,326 describes an oxygen VSA process which incorporates two stages of evacuation. Each stage is carried out by a separate vacuum machine. These machines could be of the same type or different types, e.g. volumetric and centrifugal.
U.S. Pat. No. 5,330,561 describes a 4 bed O.sub.2 VSA process with 2 vacuum pumps with the steps: adsorption, cocurrent depressurization to provide purge gas, simultaneous countercurrent evacuation and cocurrent depressurization to provide pressure equalization gas, countercurrent evacuation in two successive pumping sub-steps, countercurrent purge, receipt of aforementioned pressure equalization gas and repressurization with several combinations of product gas, feed gas, or ambient air.
U.S. Pat. No. 5,411,578 describes a 4 bed O.sub.2 VSA process with 2 vacuum pumps with the steps: adsorption, cocurrent depressurization to provide product gas, cocurrent depressurization to provide purge gas, two successive sub-steps of countercurrent evacuation, countercurrent purge, partial repressurization with product gas, followed by further repressurization with feed gas. Options include simultaneous cocurrent depressurization to provide pressure equalization gas following the provide purge step and simultaneous ambient air repressurization during the partial product repressurization step.
U.S. Pat. No. 5,246,676 describes a process for producing oxygen from air containing at least three beds and undergoing the following steps: adsorption, countercurrent evacuation including at least two successive pumping sub-steps, and product repressurization. Various options on the cycle include: cocurrent depressurization to provide purge gas to the bed under vacuum, further cocurrent depressurization to provide partial repressurization gas, and cocurrent depressurization to a storage tank from where some of the purge gas is withdrawn.
The above described dual vacuum cycles have the capability to increase single train capacity beyond 90 TPD and to reduce power consumption by allowing more time for the evacuation phase and by allowing more efficient machines to be used in the deep vacuum region. They do not, however, offer any significant improvement in adsorbent productivity relative to the 2 or 3 bed cycles.
There is clearly a need in the market place for an O.sub.2 VSA process which extends the size range of a single skid beyond 90 TPD, which substantially increases adsorbent productivity, and which integrates the machinery with the internal process flows to reduce wasted power. The present invention outlines such a process.